The liver is the largest organ in the body and is positioned in the upper right portion of the abdomen. Its main roles are the metabolism of nutrients and the detoxication and elimination of harmful substances. According to the 2011 statistics of cancer types in Japan disclosed by the Center for Cancer Control and Information Services, National Cancer Center, the number of individuals affected by liver cancer is 47,271 people. Namely, it is estimated that one out of every 35 Japanese individuals experience liver cancer. The number of individuals affected by liver cancer among other cancer types takes the 6th in place. Also, men are nearly twice as likely as women to develop liver cancer. The number of liver cancer deaths in men and women together climbed to 30,690 people and takes the 4th in place. An estimate of the number of American individuals affected by liver cancer in 2014 climbs to 33,190 people, among which approximately 23,000 people will die (Non-Patent Literature 1).
In general, primary liver cancer often refers to hepatocellular carcinoma which accounts for approximately 80% of primary liver cancer cases. However, there are other subtypes of primary liver cancer such as intrahepatic bile duct carcinoma which accounts for 10 to 20% of all primary liver cancer cases, and biliary cystadenocarcinoma which is a rarer cancer type.
The stages of liver cancer progression are specified separately for hepatocellular carcinoma and intrahepatic bile duct carcinoma in Non-Patent Literature 2. Herein, particularly, the hepatocellular carcinoma is classified into stage I (T1/N0/M0), stage II (T2/N0/M0), stage IIIA (T3a/NO/M0), stage IIIB (T3b/NO/M0), stage IIIC (T4/NO/M0), stage IVA (N1/M0), and stage IVB (M1) according to the degrees of tumor spread (T0 to T4), lymph node metastasis (N0 and N1), and distant metastasis (M0 and M1).
The 5-year relative survival rate of liver cancer differs depending on the stages of progression. According to Non-Patent Literature 1, the 5-year relative survival rate of liver cancer is reportedly 28% for tumors localized within liver (stage 1, stage 2 and some cases of stage 3), 7% for tumors found to have metastasized to a surrounding area of liver (stage IIIC and stage IVA), and 2% for tumors found to have metastasized distantly (stage IVB). Thus, the detection and treatment of liver cancer at an early stage before metastasis makes a significant contribution to improvement in the survival rate.
The treatment of liver cancer is performed mainly by 3 procedures: surgical therapy mainly involving resection and/or liver transplantation; local therapy which involves injecting a drug through centesis or performing cauterization to kill cancer; and hepatic arterial embolization. These procedures are used in combination with drug therapy or radiotherapy. Particularly, early liver cancer which is found not to metastasize to a blood vessel or an adjacent site is often cured by the partial resection of the liver (Non-Patent Literature 1). On the other hand, even if cancer is localized, liver transplantation is desirable for the cases where such resection is impossible on the ground that the tumors have a large size or are placed in proximity to a blood vessel, for example. If metastasis is found, systemic drug therapy or radiotherapy is performed (Non-Patent Literature 1).
As described in Non-Patent Literature 1, primary tests of liver cancer are inspection and palpation as well as imaging tests such as ultrasonography, CT scan, MRI scan, and angiography. For example, AFP (alpha fetoprotein) and PIVKA-II are known as tumor markers for the detection of liver cancer. The tests using these tumor markers are often performed in combination with ultrasonography. When there are findings that suspect liver cancer by these primary tests, pathological examination which involves inserting a needle into a lesion and collecting cells or tissues, which are then examined under a microscope is carried out as a secondary test.
Meanwhile, it is known that the most important leading cause of liver cancer is prolonged infection with hepatitis B or C virus. Therefore, subjects suspected of having liver cancer may be subjected to a hepatitis virus test in addition to the primary tests described above.
As shown in Patent Literatures 1 to 5, there are reports, albeit at a research stage, on methods for detecting liver cancer using the expression levels of microRNAs (miRNAs) in biological samples including blood and hepatic tissues.
Patent Literature 1 discloses a method for detecting leukemia, breast cancer, and liver cancer using miRNAs: hsa-miR-92a-3p, hsa-miR-92b-3p, hsa-miR-92a-2-5p, and hsa-miR-92b-5p in tissues as markers.
Patent Literature 2 has reported a method for diagnosing various cancers using, as markers, miRNAs such as hsa-miR-23a-3p, hsa-miR-23b-3p, hsa-miR-24-3p, hsa-miR-557, hsa-miR-564, hsa-miR-614, hsa-miR-150-3p, and hsa-miR-486-3p contained in vesicles circulating in body fluids.
Patent Literature 3 discloses a method for detecting various diseases including liver cancer using miRNAs such as hsa-miR-23b-3p, hsa-miR-30c-1-3p, hsa-miR-125a-3p, and hsa-miR-486-3p in tissues or body fluids as markers.
Patent Literature 4 discloses a method for detecting various pathological conditions including liver cancer using, as markers, miRNAs such as hsa-miR-16-5p, hsa-miR-92a-3p, hsa-miR-663a, hsa-miR-1913, and hsa-miR-625-3p, or proteins contained in vesicles circulating in body fluids.
Patent Literature 5 discloses that hsa-miR-187-5p, hsa-miR-92a-3p, hsa-miR-16-5p, and hsa-miR-30c-1-3p in plasma are markers for colorectal cancer, liver cancer, and lung cancer.